Branched polymers are polymer molecules of a finite size which are branched, often having many branches. Branched polymers differ from crosslinked polymer networks which tend towards an infinite size having interconnected molecules and which are generally not soluble. Branched polymers are usually soluble in solvents which dissolve analogous linear polymers but have the advantage that solutions of branched polymers are usually less viscous than solutions of the same concentration of the corresponding linear polymer having a similar molecular weight. Therefore solutions of branched polymers are easier to handle especially at high solids content and may be made using less solvent than solutions of linear polymers. For this reason, branched polymers are useful additives to solvent-based coatings and inks, for example and they also have many other applications. Additionally, branched polymers also have a lower melt viscosity than analogous linear polymers and are useful for improving melt processability in injection moulding, compression moulding, extrusion moulding or powder coatings.
Branched polymers may be made by a two-step process in which a linear polymer containing branching sites is subjected to a further polymerisation or modification step to form branches from the branching sites. The inherent complications of a two-step process may be unattractive and make the resulting branched polymer expensive to use. Alternatively a one-step process can be used in which a polyfunctional monomer is present to provide functionality in the polymer chain from which polymer branches may grow. However, a limitation on the use of conventional one-step processes is that the amount of polyfunctional monomer must be carefully controlled, usually to substantially less than about 0.5% w/w in order to avoid extensive cross-linking of the polymer and the formation of insoluble gels. It is very unusual to avoid crosslinking using this system, especially in the absence of a solvent as diluent and/or at high conversion of monomer to polymer.
GB-A-2294467 describes a branched polymethylmethacrylate polymer which has a molecular weight of 80,000–400,000 in which the molecular weight between the branching points is between 30,000 and 1,000,000 which includes 0.05 –0.2% of a polyfunctional monomer and <0.5 mole % of a chain transfer agent.
U.S. Pat. No. 5,767,211, published Jun. 16, 1998, describes the synthesis of multi-functional hyperbranched polymers by free-radical polymerisation of di- or tri-vinyl monomers in the presence of a chain transfer catalyst and a non-peroxide free radical initiator. The resulting polymers are oily, low Tg materials.
EP-A-103199 describes copolymers of t-butyl acrylate with 0.1–3% polyfunctional acrylate and 1–30% of functional comonomer made by solution polymerisation in the presence of a chain transfer agent. The functional comonomer provides an active cross-linking site used to form a coating composition crosslinked by condensation chemistry.
U.S. Pat. No. 4,880,889 describes a pre-crosslinked soluble polymer containing 10–60% of OH-functionalised monomer, 5–25% of a monomer with at least 2 olefinically unsaturated double bonds and 15–82% of further monofunctional monomers. The polymer composition is made by a solution polymerisation process in organic solvent at a low polymerised solids content of about 50% in order to produce an ungelled copolymer, using >0.5% of a polymerisation regulator. The polymers are used in crosslinked coatings where the OH group is reacted with melamine-formaldehyde crosslinkers. U.S. Pat. No. 4,988,760 and U.S. Pat. No. 5,115,064 define similar compositions which include functionalised monomers having different cross-linkable groups which include carboxyl and isocyanate.